1. Field of the Invention
The present invention relates to an optical demultiplexing module, as well as an optical multiplexing module that are used with amplifiers in fiber optic communication. The invention also relates to housings for such optical demultiplexing and multiplexing modules.
2. Description of the Related Art
Optical amplifiers capable of direct amplification of optical signals are becoming the subject of active development efforts since they can be used as repeaters to compensate for the attenuation of optical signals caused by transmission loss and other factors of optical fibers. Of a particular interest are optical amplifiers that use rare-earth doped fibers as amplifying media because of their small polarization-dependence and low-noise characteristics.
Such optical amplifiers can be implemented by two methods. One method is to combine rare-earth doped fibers with fiber optic couplers and the second method is to combine the fibers with optical devices such as a multiplexer, a beam splitter and ah optical isolator. The present invention concerns the second method of implementation. A conventional system that adopts this method is shown in FIG. 9, where signal light is combined with pumping light by means of a multiplexer 53 and the combined light is admitted into a rare-earth doped fiber 10 (for details, see Unexamined Published Japanese Patent Application (kokai) Hei 4-128718). Another conventional system uses a prism to combine the signal light with the pumping light (for details, see C-262 in the Collected Papers Read at the Spring Conference of The Institute of Electronics, Information and Communication Engineers, 1992).
If the optical devices used in an optical amplifier are largely dependent on polarization and wavelength, the characteristics of the amplifier per se are adversely affected to a large extent. To avoid this problem, the optical devices such as a multiplexer and a wavelength selecting filter are adapted to admit light at a smaller incident angle or otherwise modified so that they have no dependence on polarization or wavelength.
In the system configuration shown in FIG. 9, an optical fiber 51 is mounted at an angle with respect to the multiplexer 53 and this is in order to avoid the dependence on polarization and wavelength by insuring that the angle of incidence .theta. on the multiplexer 53 is sufficiently small. However, it is a disadvantage for production purposes that various optical devices are fixed so as to be inclined with respect to the sides of the housing during assembly because the production consumes a great deal of time.
If a prism is used as a multiplexer, other devices can be fixed in a direction normal to the sides of the housing but then a special treatment must be conducted on the devices. This contributes to a high manufacturing cost.
The conventional approaches have had a common problem in that the individual optical devices have to be assembled one by one into a single housing.